<?php
//implode(string $separator, array $array): string
//implode — 用字符串连接数组元素

$arr = ['piece1', 'piece2', 'piece3', 'piece4'];
$str = implode(',', $arr);
print_r($str);
echo PHP_EOL;
$str2 = join(',', $arr);
print_r($str2);
